This invention relates generally to migration imaging, and more specifically to an improved migration imaging process.
Migration imaging systems capable of producing high quality images of high optical density, continuous tone and high resolution, have been developed. Such migration imaging systems are disclosed, for example, U.S. Pat. No. 3,909,262 which issued Sept. 30, 1975 and U.S. Pat. No. 3,975,195 which issued Aug. 17, 1976, the disclosures of both being incorporated herein in their entirety. In a typical embodiment of migration imaging systems, an imaging member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material was originally in the form of a fracturable layer contiguous the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer.
The expression "softenable" as used herein in intended to mean any material which can be rendered more permeable thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein, means any layer or material which is capable of breaking up during development, thereby permitting portions of said layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members of the present invention. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers may be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members of the inventive system.
The expression "contiguous" as used herein is intended to mean in actual contact; touching; also, near, though not in contact; and adjoining, and is intended to generically describe the relationship of the fracturable layer of marking material in the softenable layer, vis-a-vis, the surface of the softenable layer spaced apart from the substrate.
The expression "sign retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the image formed on the migration imaging member correspond to the dark and light areas of the image on the original.
The expression "signal reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the image on the original and the light areas of the image formed on the migration imaging member correspond to the dark areas of the image on the original.
The expression "contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this application by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. While contrast density is measured by diffuse densitometers in this application, it should be noted that measurement by specular densitometers gives substantially similar results.
There are various other systems for forming such images, where non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patent, which also discloses a variety of methods which may be used to form latent images upon migration imaging members.
Various means for developing the latent images in the novel migration imaging system may be used. These development methods include solvent wash-away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash-away or meniscus development method, the migration marking material in the light-struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. This region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore the image-sense of the developed image is sign-reversed, i.e. positive to negative or vice versa. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat, or vapor softening developing modes, the migration marking material in the light-struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6-0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. This region tube exhibits maximum optical density (D.sub.max). Therefore, the image sense of the heat or vapor developed images is sign-retaining, i.e. positive-to-positive or negative-to-negative.
Techniques have been devised to permit sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be found in U.S. Pat. No. 3,795,512.
For many imaging applications, such as lithographic intermediate film in the graphic arts industry, it is desirable to produce negative images from a positive original or positive images from a negative original i.e. sign-reversing imaging, preferably with low mininum optical density. Although the meniscus or solvent wash-away development method produces sign-reversed images with low minimum optical density, it involves removal of materials from the migration imaging member, leaving the migration image largely or totally unprotected from abrasion. Although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step is impractically costly and inconvenient for the end users. Additionally, disposal of the effluents washed from the migration imaging member during development is very costly. While heat or vapor development methods are preferred because they are rapid, essentially dry and produce no liquid effluents, the image sense of the heat or vapor developed images is sign-retaining and the minimum optical density is quite high. Therefore, there is a continuing need for a simple, inexpensive, and usable imaging member capable of sign-reversing imaging with essentially dry development methods and preferably giving low minimum optical density.
The background portions of an imaged member may sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and the softenable layer softened by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85-90% of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 after vapor exposure, compared with an initial value of 1.8 to 1.9. In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25-0.35 range. Thus the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image may be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer. Although the minimum optical density (D.sub.min) of images using such techniques is much reduced, there is generally a concurrent drastic reduction in the maximum optical density (D.sub.max) (since these areas consist of marking material particles which have migrated substantially in depth) and consequently the contrast density (D.sub.max -D.sub.min) is also low. There is also usually a substantial reduction in the resolving power of the film, because of the agglomeration of the marking material particles.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the agglomerate towards a shape of lower energy, such as a sphere.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the film and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in the aforementioned U.S. Pat. No. 3,909,262. However, because the migration imaging mechanisms for each development method are different and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles etc., application of an overcoat to the softenable layer often causes changes in the delicate balance of these processes, and results in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density is degraded.
In addition, many overcoatings do not prevent blocking when migration imaging members are stacked or wound into rolls. In addition, for applications where migration imaging members are utilized for composing lithographic intermediates wherein imaged migration imaging members are temporarily secured by adhesive tape to a substrate and thereafter reused, very often the migration imaging member is damaged by removal of the adhesive tape and is rendered unsuitable for reuse. This damage generally takes two forms. First, many overcoats do not adhere well to the softenable layer of the migration imaging member and can be separated by flexing or easily separated or removed entirely from the softenable layer upon removal of the adhesive tape, thereby eliminating further abrasion resistance. Secondly, the softenable layer which contains the photoactive particles often separates from the substrate upon removal of the adhesive tape. Therefore, the overcoat should not only adhere well to the softenable layer but should also have abhesive properties to release the adhesive tape to prevent damage to the migration imaging member.
Therefore, there continues to be a need for an improved migration imaging process. Additionally, there is a need for an improved migration imaging process capable of producing sign-reversed images having high contrast density and low D.sub.min, which exhibit greater resistance to the adverse effects of finger prints, blocking, softenable layer/overcoating layer interface failure, and abrasion, and which can survive adhesive tape tests.